The present invention relates to an overcurrent protection PTC element having a positive temperature coefficient (PTC) that increases its resistance drastically in a certain range of temperature.
The use of PTC devices to control current flow in an electrical circuit and compensate for the effects of temperature is known. An example of conventional PTC elements, Japanese Patent Publication No. 33707/1975 discloses a temperature-sensitive conductive composition wherein carbon black powder having a generally spherical particle shape and an average particle diameter of 0.08.mu.-200.mu. is blended with crystalline polymer. The publication teaches that a PTC element using large, spherical conductive carbon black particles exhibits excellent PTC characteristics even in a low resistance range.
Japanese Patent Publication No. 3322/1989 discloses an electrical circuit protection device wherein carbon black blended with crystalline polymer has particle diameter (D) of 20 m.mu.-150 m.mu. with the ratio S/D of specific surface area S (m.sup.2 /g) to particle diameter D (m.mu.) being not more than 10. This publication teaches that it is desirable to use carbon black with a particle diameter of less than 100 m.mu., because carbon black of large particle diameter makes it difficult to obtain a PTC composition that has both low volume resistivity and sufficient PTC characteristics.
Japanese Patent Publication Laid-Open No. 80201/1985 discloses a conductive material with heat sensitive resistance which is a mixture of a crystalline polymer and a carbon black having an average particle size of less than 0.08.mu., the carbon black having a weight of between about 25 to about 60% of the crystalline polymer. This publication teaches that carbon black with average particle diameter more than 0.08.mu. is not desirable because the resistance value of a conductive material with heat sensitive resistance using such would be too high in the normal temperature range.
When voltage decrease in a circuit is considered, it is desirable for an overcurrent protection element to have low resistance value and also because of the recent trend for making electrical devices compact and using high density circuits, such element should be small in size.
Where a PTC composition is used to make a small, low resistance overcurrent protection element, the volume resistivity of the PTC composition must be low.
Dispersing conductive particles in polymer is a known method for making a polymer conductive, and if conductive carbon black, such as Ketjen Black EC (manufactured and sold under that name by Nippon EC Co., LTD.), is used for that purpose, a very low resistance value is possible. However, this type of composition cannot be used for overcurrent protection, because its resistance value increases very little relative to its initial resistance at normal operating temperatures, even in its maximum PTC range. The reason for this is thought to be that because the conductive particles are small, their specific surface area is large, causing them to aggregate with such strength as makes it difficult for them to disperse evenly in a polymer. Unevenly dispersed carbon black particles form continuous conductive paths in the polymer and while this improves conductivity of the material, it makes it impossible to effectively separate the carbon black particles in these conductive paths from each other during polymer thermal expansion so that proper PTC characteristics cannot be achieved.
The carbon black described in the Japanese Patent Publication No. 3322/1989 and Japanese Patent Publication Laid-Open No. 80201/1985 is of smaller particle size and larger in specific surface area than that of the Ketjen Black EC previously described conductive carbon black. Nonetheless some dispersion of such throughout a polymer is possible. However, when the amount of carbon black is increased to reduce the volume resistivity of the PTC composition, there unavoidably occurs the formation of continuous conductive paths which are not broken during thermal expansion. As a result, the more that volume resistivity is reduced, the smaller the PTC characteristics become, making it impossible to maintain the PTC characteristics necessary for overcurrent protection.
Japanese Patent Publication No. 33707/1975 states that it is possible to obtain a PTC composition having low resistance value and superior characteristics by using carbon black made of generally spherical particles having average particle sizes a range of about 0.08.mu. to about 200.mu.. It seems that such conductive particles are easily dispersed in polymer and effectively separated at the time of thermal expansion of the polymer. However, the performance of a PTC composition using such conductive particles is no better than those disclosed in Japanese Patent Publication No. 3322/1989 and Japanese Patent Publication Laid-Open No. 80201/1985.
It is clear that whenever conductive particles as described in Japanese Patent Publication No. 33707/1975, Japanese Patent Publication No. 3322/1989 or Japanese Patent Publication Laid-Open No. 80201/1985 are used for a PTC element, there is a limitation in how small the PTC element can be made and how far the resistance value can be lowered.